Origin of the Crescent Moon Spring in the Gobi Desert of northwestern China, based on understanding groundwater recharge

Abstract The Crescent Moon Spring is a unique scenic spot located in an aeolian sand environment of northwestern China. The water level of this spring has continuously declined in recent decades, causing both the government and many scholars great concern. To better understand its hydrologic structure, this study investigated the origin of the spring and its recharge sources based on the hydrochemical and isotopic (18O, 2H and 3H) composition of 56 water samples collected along the groundwater flow path. The results indicate the spring is a window into the groundwater table cut out of low-lying terrain in sand dunes, with the geographic and sedimentary conditions affecting its genesis and hydrochemistry. The spring and shallow sedimentary aquifers are recharged primarily by river water, while deep bedrock aquifers are recharged by deep lateral groundwater flow via fault zones in the piedmont of the Qilian Mountains. The results of recharge altitude calculations demonstrate a possible groundwater supply from deep bedrock aquifers to the south of spring. The recorded environmental 3H content varied from 11 to 40 T.U., indicating the groundwater is primarily recharged by glacier meltwater from modern precipitation, with only a small fraction from pre-modern times. End-member analysis using Cl- and δ18O shows recharge ratios of influent water from the Dang River and deep groundwater represent about 43 and 22% of the lake water, respectively, with artificial recharge accounting for the remaining 35%. Regional hydrogeological conditions, arid climate, and sand deposition have negative influences on the maintenance of lake water levels, but observed regional groundwater level declines resulting from the construction of the Dang River Reservoir, seepage from drainage canals, and excessive exploitation of groundwater are the most important factors contributing to the ecological crisis in the study area. These results provide insights into the origin and recharge sources of the Crescent Moon Spring that will guide efforts to preserve the water level.